Direct conversion receivers are known in the art. In a direct conversion receiver, the received carrier frequency signal with the data signal modulated on it is mixed with a signal from a local oscillator having the same frequency as the carrier frequency. The signal product of these two signals is a signal having components at the base band and at two times the carrier frequency. The high frequency component can be filtered using a low pass filter, such that the data signal remains in the base band signal. The base band signal is then processed to extract the data encoded in the transmitted signal.
One drawback of direct conversion receivers is that the local oscillator can radiate a leakage signal that may be stronger than the signal transmitted and received by the antenna. In this configuration, it is possible for the local oscillator signal to be received at the antenna of the direct conversion receiver or at intermediate points between the antenna and the mixer. If this condition occurs, the radiated local oscillator signal is received and mixed with the local oscillator signal that is provided directly at the input of the mixer, which creates a significant DC signal component in the output from the mixer.
In order to prevent the generation of a DC signal from such feedback, shielding or filtering can be performed. Likewise, it is also possible to offset the DC signal by applying a DC signal that is equal to that created by the leakage signal from the local oscillator and having a reverse polarity. Both shielding and offsetting the DC signal require additional costs, such as the cost and expense of shielding, the additional weight of shielding, the additional power required to generate the DC offset, and other similar costs.
Thus, while direct conversion receivers have some utility, the additional costs required to shield the direct conversion receiver antenna from the local oscillator or to offset the DC signal can significantly increase the weight and costs of the direct conversion receiver. The increased weight and cost of the direct conversion receiver limits the types of applications in which the direct conversion receiver can be effectively used.